1. Field of the Invention
The present invention relates to a circuit for detecting voltage variations in relation to a set value, for devices comprising error amplifiers. In particular, the present invention is especially suitable for use on boost-type switch power supply circuits for detecting and limiting overvoltages in relation to a set value.
2. Discussion of the Related Art
Power supply circuits convert an alternating-current ("AC") voltage into a given direct-current ("DC") output voltage. Boost-type switch power supply circuits use a control circuit to control the on-off operation of a power transistor, which, when on, permits the charging of an inductor to the current required for achieving a given output voltage (greater than the input voltage) and which, when off, allows the discharging of the inductor, via a diode, to a load. An output capacitor is provided for reducing the ripple produced by the switching of the power transistor.
A known circuit of the aforementioned type is shown by way of reference in FIG. 1, wherein the power supply circuit is indicated, as a whole, by 1 and substantially comprises a rectifying bridge 2 having input terminals 3, which receives a sinusoidal AC input voltage V.sub.M, and output terminals 5 and 6. Bridge output terminal 5 is connected, via resistor 5a, to the input pin 7 of a control circuit 8, an integrated circuit. Terminal 6 is connected to a reference potential line 9 (ground). Bridge output terminal 5 is also connected to one terminal of winding 10 of transformer 11. The other terminal of winding 10 is connected to the anode of diode 12. A capacitor 13, load 14, and a divider 15, which includes resistors 16 and 17 and node 18 there between, are connected in parallel with one end of the parallel combination connected to the cathode of diode 12 and the other end grounded. Cathode 12a of diode 12 forms the output terminal of power supply circuit 1. Node 18 is connected to input pin 19 of control circuit 8.
Transformer 11 comprises a second winding 20 having one terminal grounded and the other terminal connected to pin 21 of control circuit 8, via resistor 20a, and also connected to the anode of diode 22. The cathode of diode 22 is connected to ground via a capacitor 23, and is also connected to pin 24 of control circuit 8.
Control circuit 8 substantially comprises an error amplifier 27 having the positive input connected to reference voltage V.sub.R, and the negative input connected to input pin 19 of control circuit 8. A compensating capacitor 28 is connected to input pin 19 and pin 29. Capacitor 28 connects the negative input of amplifier 27 with the amplifier's output.
Compensating capacitor 28 is external to control circuit 8. The output of error amplifier 27 is connected to one input of a multiplier stage 30, which is within control circuit 8; second input of multiplier stage 30 is connected to pin 7. The output of multiplier stage 30 is connected to control stage 31, which is also connected to pin 24. The output of control stage 31 is connected, via pin 38, to the gate terminal of a power MOS transistor 34. Control stage 31 comprises the control logic for turning transistor 34 on and off and also provides for driving transistor 34. The drain terminal of transistor 34 is connected to the anode of diode 12, and the source terminal is connected to pin 35 of control circuit 8. Finally, a resistor 36 is provided between the source terminal of transistor 34 and ground, and control circuit 8 is connected to ground line 9, via pin 37.
A major drawback of the FIG. 1 circuit is that it fails to instantly eliminate sharp overvoltages at the output 12a, caused, for example, by a sharp variation in load 14. This is due to the limited response of control circuit 8, which must be so designed as to be unaffected by variations in output voltage of twice the frequency of input voltage V.sub.M.
To overcome the above drawback, one potential solution is to provide a second divider (not shown) at the output 12a, in parallel to divider 15, for receiving a signal proportional to the instantaneous value of output voltage V.sub.o. The tap of the additional divider is connected to the input of a hysteresis comparator built into control circuit 8 and which compares the incoming signal with a reference voltage and, in the event that the compared signal exceeds the reference voltage, supplies a turn-off signal to control stage 31, thus turning transistor 34 off until output voltage V.sub.o is restored to a correct value.
Such a solution, however, requires a special pin on control circuit 8 for receiving the signal from the second divider. As such, it cannot be applied to all control circuits 8, particularly those mounted in eight-pin packages in which no pins are available for the purpose. Moreover, in any event, it requires an additional pin, always a scarce resource.
It is an object of the present invention to provide a circuit for detecting voltage variations and which does not require an additional pin for receiving external signals from the output.